U.S. patent application number 12/947904 was filed with the patent office on 2011-06-23 for method and apparatus to reduce internal pressure caused by an arcing fault in an electrical enclosure.
Invention is credited to Ronald D. Hartzel, Paul K. Parker, JOHN J. SHEA, James E. Smith.
Application Number | 20110149478 12/947904 |
Document ID | / |
Family ID | 44150746 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110149478 |
Kind Code |
A1 |
SHEA; JOHN J. ; et
al. |
June 23, 2011 |
METHOD AND APPARATUS TO REDUCE INTERNAL PRESSURE CAUSED BY AN
ARCING FAULT IN AN ELECTRICAL ENCLOSURE
Abstract
An electrical enclosure includes a housing having a first end,
an opposite second end, and a plurality of sides disposed
therebetween to define an internal volume; an electrical busway
having a plurality of electrical bus members; a plurality of
phase-to-phase arc length limiters, each of the phase-to-phase arc
length limiters being electrically connected to a corresponding one
of the electrical bus members, each of the phase-to-phase arc
length limiters having a first edge and a second edge, the first
edge establishing a first gap to an adjacent one of the
phase-to-phase arc length limiters; and a phase-to-ground arc
length limiter electrically connected to the housing, the
phase-to-ground arc length limiter having a number of members
structured to attach an arc, each of the number of members having a
number of arc attachment portions establishing a second gap to the
second edge of the phase-to-phase arc length limiters.
Inventors: |
SHEA; JOHN J.; (Pittsburgh,
PA) ; Hartzel; Ronald D.; (Butler, PA) ;
Smith; James E.; (Pittsburgh, PA) ; Parker; Paul
K.; (Pine Township, PA) |
Family ID: |
44150746 |
Appl. No.: |
12/947904 |
Filed: |
November 17, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61287455 |
Dec 17, 2009 |
|
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Current U.S.
Class: |
361/601 |
Current CPC
Class: |
H02B 1/20 20130101; H02B
13/025 20130101 |
Class at
Publication: |
361/601 |
International
Class: |
H02B 1/00 20060101
H02B001/00 |
Claims
1. An electrical enclosure comprising: a housing comprising a first
end, an opposite second end, and a plurality of sides disposed
therebetween to define an internal volume; an electrical busway
comprising a plurality of electrical bus members; a plurality of
phase-to-phase arc length limiters, each of the phase-to-phase arc
length limiters being electrically connected to a corresponding one
of said electrical bus members, each of the phase-to-phase arc
length limiters having a first edge and a second edge, the first
edge establishing a first gap to an adjacent one of said
phase-to-phase arc length limiters; and a phase-to-ground arc
length limiter electrically connected to said housing, said
phase-to-ground arc length limiter comprising a number of members
structured to attach an arc, each of the number of members having a
number of arc attachment portions establishing a second gap to the
second edge of the phase-to-phase arc length limiters.
2. The electrical enclosure of claim 1 wherein the plurality of
electrical bus members is three copper busses.
3. The electrical enclosure of claim 1 wherein each of the
electrical bus members includes a cable terminal portion separated
from a corresponding one of the phase-to-phase arc length
limiters.
4. The electrical enclosure of claim 3 wherein one of the sides of
said housing supports a bracket carrying a plurality of insulators,
each of said insulators supporting the cable terminal portion of a
corresponding one of said electrical bus members.
5. The electrical enclosure of claim 3 wherein the cable terminal
portion is structured to receive a number of power cables.
6. The electrical enclosure of claim 1 wherein said phase-to-phase
arc length limiters and said phase-to-ground arc length limiter are
made of steel.
7. The electrical enclosure of claim 1 wherein said first gap is
about four inches.
8. The electrical enclosure of claim 1 wherein said second gap is
about four inches.
9. The electrical enclosure of claim 1 wherein one of the sides of
said housing comprises a side channel; and wherein said
phase-to-ground arc length limiter is electrically connected to the
side channel.
10. The electrical enclosure of claim 1 wherein the number of arc
attachment portions is a third edge of each of the number of
members of said phase-to-ground arc length limiter is parallel to
the third edge of each of the other number of members and
establishes the same second gap to the second edge of each of the
phase-to-phase arc length limiters.
11. The electrical enclosure of claim 10 wherein the same second
gap is about four inches.
12. The electrical enclosure of claim 1 wherein the corresponding
one of said electrical bus members includes a conductor separated
from a corresponding one of the phase-to-phase arc length limiters;
wherein current normally flows in the corresponding one of said
electrical bus members and in the conductor separated from the
corresponding one of the phase-to-phase arc length limiters, but
not in the corresponding one of the phase-to-phase arc length
limiters; and wherein each of the phase-to-phase arc length
limiters direct fault current flowing in the corresponding one of
said electrical bus members in an arc between an adjacent pair of
the phase-to-phase arc length limiters, but not in said
conductor.
13. An electrical enclosure comprising: a housing comprising a
first end, an opposite second end, and a plurality of sides
disposed therebetween to define an internal volume; an electrical
busway comprising a plurality of electrical bus members; and a
plurality of phase-to-phase arc length limiters, each of the
phase-to-phase arc length limiters being electrically connected to
a corresponding one of said electrical bus members, each of the
phase-to-phase arc length limiters having an edge establishing a
gap to an adjacent one of said electrical bus members, wherein the
corresponding one of said electrical bus members includes a
conductor separated from a corresponding one of the phase-to-phase
arc length limiters, wherein current normally flows in the
corresponding one of said electrical bus members and in the
conductor separated from the corresponding one of the
phase-to-phase arc length limiters, but not in the corresponding
one of the phase-to-phase arc length limiters, and wherein each of
the phase-to-phase arc length limiters direct fault current flowing
in the corresponding one of said electrical bus members in an arc
between an adjacent pair of the phase-to-phase arc length limiters,
but not in said conductor.
14. The electrical enclosure of claim 13 wherein the plurality of
electrical bus members are three electrical bus members; wherein
the plurality of phase-to-phase arc length limiters are three
phase-to-phase arc length limiters; wherein each of the three
electrical bus members includes a cable terminal portion; wherein a
first one of the electrical bus members and the cable terminal
portion thereof form a first L-shape; wherein a second one of the
electrical bus members and the cable terminal portion thereof form
a T-shape; and wherein a third one of the electrical bus members
and the cable terminal portion thereof form a second L-shape;
wherein the T-shape is disposed between the first L-shape and the
second L-shape; and wherein the cable terminal portions of the
first one and the third one of the electrical bus members form one
side of the first L-shape and the second L-shape, respectively,
disposed proximate but separated from the edge of a corresponding
one of the phase-to-phase arc length limiters.
15. The electrical enclosure of claim 14 wherein the phase-to-phase
arc length limiters comprise a member having a generally
rectangular shape; wherein the member has an edge defining the edge
of the corresponding one of the phase-to-phase arc length limiters;
wherein the member is disposed above a corresponding one of the
first L-shape, the T-shape and the second L-shape; wherein the
cable terminal portions of the first one and the third one of the
electrical bus members are disposed proximate but separated from
the edge of the member of the corresponding one of the
phase-to-phase arc length limiters; and wherein the cable terminal
portion of the second one of the electrical bus members bisects but
is separated from the member of the corresponding one of the
phase-to-phase arc length limiters.
16. An electrical enclosure comprising: a housing comprising a
first end, an opposite second end, and a plurality of sides
disposed therebetween to define an internal volume; an electrical
busway comprising a number of electrical bus members; and a
phase-to-ground arc length limiter electrically connected to said
housing, said phase-to-ground arc length limiter comprising a
number of members, each of the number of members having a number of
arc attachment portions establishing a second gap to the second
edge of the number of electrical bus members.
17. The electrical enclosure of claim 16 wherein the number of
electrical bus members is one conductive bus member.
18. The electrical enclosure of claim 16 wherein the number of
electrical bus members is a plurality of conductive bus
members.
19. A method of reducing pressure caused by an arcing fault, said
method comprising: employing an electrical busway comprising a
plurality of electrical bus members; employing a plurality of
phase-to-phase arc length limiters, each of the phase-to-phase arc
length limiters being electrically connected to a corresponding one
of said electrical bus members, each of the phase-to-phase arc
length limiters having an edge establishing a gap to an adjacent
one of said phase-to-phase arc length limiters; including with the
corresponding one of said electrical bus members a conductor
separated from a corresponding one of the phase-to-phase arc length
limiters; providing normal current flow in the corresponding one of
said electrical bus members and in the conductor separated from the
corresponding one of the phase-to-phase arc length limiters; but
not in the corresponding one of the phase-to-phase arc length
limiters; and directing fault current flowing in each of the
phase-to-phase arc length limiters and in the corresponding one of
said electrical bus members in an arc between an adjacent pair of
the phase-to-phase arc length limiters, but not in said
conductor.
20. The method of claim 19 further comprising: employing the edge
of each of the phase-to-phase arc length limiters as a first edge;
providing a second edge for each of the phase-to-phase arc length
limiters; grounding a phase-to-ground arc length limiter; and
providing a number of members with the phase-to-ground arc length
limiter, each of the number of members having a number of arc
attachment portions establishing a gap to the second edge of the
phase-to-phase arc length limiters.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/287,455, filed Dec. 17, 2009.
BACKGROUND
[0002] 1. Field
[0003] The disclosed concept pertains generally to electrical
enclosures and, more particularly, to such electrical enclosures
structured to resist internal arcing faults. The disclosed concept
also pertains to methods of reducing pressure caused by arcing
faults.
[0004] 2. Background Information
[0005] Electrical equipment such as, for example and without
limitation, electrical busways, relays, circuit interrupters,
electric meters and transformers, are typically housed within an
electrical enclosure such as, for example, a housing, such as a
box, cabinet, module or compartment, to protect the electrical
equipment.
[0006] Electrical enclosures can enclose a wide range of electrical
equipment, such as, for example and without limitation, medium
voltage motor starter(s), low voltage switchgear, low voltage motor
control center(s), low voltage switchboard(s), low voltage
panelboard(s), and medium and/or low voltage transfer switches.
[0007] Switchgear typically includes a combination of an electrical
busway and electrical disconnects, fuses and/or circuit breakers
employed to electrically connect and disconnect electrical
equipment. As one non-limiting example, switchgear includes an
assembly of one or more motor starters that can also contain
circuit breakers and fused switches. Example switchgear devices
include, but are not limited by, a circuit interrupter, such as a
circuit breaker (e.g., without limitation, low voltage; medium
voltage; high voltage); a motor controller/starter; and/or any
suitable device which carries or transfers current from one place
to another.
[0008] Arc resistant switchgear is intended to mitigate the effects
of internal arcing or arc flash outside of the electrical enclosure
(e.g., without limitation, low voltage; medium voltage; high
voltage). Unintended internal arcing faults can occur from a
variety of causes (e.g., without limitation, accidental dropping of
tools; the presence of animals; insulation failure).
[0009] Excessive pressure resulting from an unintended internal
arcing fault can cause damage to the electrical enclosure resulting
in hot gases, molten copper and steel escaping the electrical
enclosure and creating a potential hazard. Hence, it is highly
desirable to reduce internal pressures generated during an arcing
fault, in order to reduce the chance of hot gases escaping from the
electrical enclosure.
[0010] FIG. 1 shows a three-phase electrical busway 2 including
back-connected (e.g., electrically connected to a surface facing
the rear of the corresponding electrical enclosure (not shown))
power cables 4. The fault current path 6 in, for example, the bus
members 12,10 provides a generally downward (with respect to FIG.
1) J.times.B force 14 that elongates the phase-to-phase arc 16.
Similarly, another J.times.B force 15 elongates the other
phase-to-phase arc 18 between bus members 10,8. Convective forces
20 attempt to lift (with respect to FIG. 1) the arcs 16,18
resulting in an angled downward (with respect to FIG. 1) direction
22 thereof as shown. The elongated arcs 16,18 increase the
corresponding arc voltage and arc power and, thus, increase the
pressure created in the corresponding electrical enclosure (not
shown). Although not shown in FIG. 1, arcs (not shown) on the outer
phases 24,26 attach to adjacent electrical enclosure walls (not
shown).
[0011] There is room for improvement in electrical enclosures
including an electrical busway.
[0012] There is also room for improvement in methods of reducing
pressure caused by arcing faults.
SUMMARY
[0013] These needs and others are met by embodiments of the
disclosed concept which employ a plurality of phase-to-phase arc
length limiters each electrically connected to a corresponding
electrical bus member and/or a phase-to-ground arc length limiter
electrically connected to a housing.
[0014] In accordance with one aspect of the disclosed concept, an
electrical enclosure comprises: a housing comprising a first end,
an opposite second end, and a plurality of sides disposed
therebetween to define an internal volume; an electrical busway
comprising a plurality of electrical bus members; a plurality of
phase-to-phase arc length limiters, each of the phase-to-phase arc
length limiters being electrically connected to a corresponding one
of the electrical bus members, each of the phase-to-phase arc
length limiters having a first edge and a second edge, the first
edge establishing a first gap to an adjacent one of the
phase-to-phase arc length limiters; and a phase-to-ground arc
length limiter electrically connected to the housing, the
phase-to-ground arc length limiter comprising a number of members
structured to attach an arc, each of the number of members having a
number of arc attachment portions establishing a second gap to the
second edge of the phase-to-phase arc length limiters.
[0015] In accordance with another aspect of the disclosed concept,
an electrical enclosure comprises: a housing comprising a first
end, an opposite second end, and a plurality of sides disposed
therebetween to define an internal volume; an electrical busway
comprising a plurality of electrical bus members; and a plurality
of phase-to-phase arc length limiters, each of the phase-to-phase
arc length limiters being electrically connected to a corresponding
one of the electrical bus members, each of the phase-to-phase arc
length limiters having an edge establishing a gap to an adjacent
one of the electrical bus members, wherein the corresponding one of
the electrical bus members includes a conductor separated from a
corresponding one of the phase-to-phase arc length limiters,
wherein current normally flows in the corresponding one of the
electrical bus members and in the conductor separated from the
corresponding one of the phase-to-phase arc length limiters, but
not in the corresponding one of the phase-to-phase arc length
limiters, and wherein each of the phase-to-phase arc length
limiters direct fault current flowing in the corresponding one of
the electrical bus members in an arc between an adjacent pair of
the phase-to-phase arc length limiters, but not in the
conductor.
[0016] In accordance with another aspect of the disclosed concept,
an electrical enclosure comprises: a housing comprising a first
end, an opposite second end, and a plurality of sides disposed
therebetween to define an internal volume; an electrical busway
comprising a number of electrical bus members; and a
phase-to-ground arc length limiter electrically connected to the
housing, the phase-to-ground arc length limiter comprising a number
of members, each of the number of members having a number of arc
attachment portions establishing a second gap to the second edge of
the number of electrical bus members.
[0017] In accordance with another aspect of the disclosed concept,
a method reduces pressure caused by an arcing fault. The method
comprises: employing an electrical busway comprising a plurality of
electrical bus members; employing a plurality of phase-to-phase arc
length limiters, each of the phase-to-phase arc length limiters
being electrically connected to a corresponding one of the
electrical bus members, each of the phase-to-phase arc length
limiters having an edge establishing a gap to an adjacent one of
the phase-to-phase arc length limiters; including with the
corresponding one of the electrical bus members a conductor
separated from a corresponding one of the phase-to-phase arc length
limiters; providing normal current flow in the corresponding one of
the electrical bus members and in the conductor separated from the
corresponding one of the phase-to-phase arc length limiters; but
not in the corresponding one of the phase-to-phase arc length
limiters; and directing fault current flowing in each of the
phase-to-phase arc length limiters and in the corresponding one of
the electrical bus members in an arc between an adjacent pair of
the phase-to-phase arc length limiters, but not in the
conductor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A full understanding of the disclosed concept can be gained
from the following description of the preferred embodiments when
read in conjunction with the accompanying drawings in which:
[0019] FIG. 1 is an isometric view of a three-phase electrical
busway including back-connected power cables.
[0020] FIGS. 2 and 3 are isometric views of three-phase electrical
busways including arc length limiter configurations in accordance
with other embodiments of the disclosed concept.
[0021] FIG. 4 is an isometric view of the three-phase electrical
busway and the phase-to-phase arc length limiters of FIG. 2.
[0022] FIG. 5 is a vertical elevation view of the three-phase
electrical busway and the phase-to-phase arc length limiters of
FIG. 2 as mounted in an electrical enclosure.
[0023] FIG. 6 is an isometric view of an electrical enclosure
including the arc length limiter configuration of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] As employed herein, the term "number" shall mean one or an
integer greater than one (i.e., a plurality).
[0025] As employed herein, the term "electrical bus" or "electrical
bus member" or "bus member" means a substantially rigid or rigid
conductor or a flexible conductor or another suitable power
conductor which carries or transfers voltage, current and/or
power.
[0026] As employed herein, the term "electrical busway" means a
plurality of electrical bus members for an electrical enclosure.
The electrical bus members receive electrical power from, for
example, a utility or other suitable power source.
[0027] As employed herein, the statement that two or more parts are
"coupled" together shall mean that the parts are joined together
either directly or joined through one or more intermediate
parts.
[0028] Directional phrases used herein, such as, for example, top,
bottom, front, back, left, right, upper, lower and derivatives
thereof, relate to the orientation of the elements shown in the
drawings and are not limiting upon the claims unless expressly
recited therein.
[0029] For purposes of illustration, embodiments of the disclosed
concept will be described as applied to switchgear enclosures,
although it will become apparent that they could also be applied to
other types of electrical enclosures (e.g., without limitation,
electrical distribution centers; motor control centers; meter
centers; modules or compartments of larger electrical enclosures;
low, medium or high voltage enclosures; panelboards; switchboards;
load centers; transfer switches).
[0030] In arc resistant equipment, a relatively greater distance
between phases and between phase and ground provides higher arc
voltage, arc power and arc pressure. Embodiments of the disclosed
concept provide arc length limiters that reduce the distance
between phases and between phase and ground in an environmentally
friendly manner (e.g., without limitation, without resort to use of
an insulating gas, such as sulfur hexafluoride (SF.sub.6)).
[0031] Referring to FIG. 2, an electrical busway 100 includes a
plurality of electrical bus members 102,104,106. A plurality of
phase-to-phase arc length limiters 108,110,112 are electrically
connected to the electrical bus members 102,104,106, respectively.
As best shown with the phase-to-phase arc length limiters
108,110,112 in FIG. 4, each of the phase-to-phase arc length
limiters has a first edge 114 (limiter 110 has two edges 114) and a
second edge 116. The first edge 114 establishes a first gap 118 to
an adjacent one of the phase-to-phase arc length limiters
108,110,112. A phase-to-ground arc length limiter 120 is
electrically connected to a housing 122 of an electrical enclosure
124 (as shown in FIG. 5). The phase-to-ground arc length limiter
120 includes a number of members, such as the example plates 126.
Each of the example plates 126 has a number of arc attachment
portions, such as an example third edge 128 establishing a second
gap 130 to the second edge 116 of the phase-to-phase arc length
limiters 108,110,112. Although one or more example plates 126 are
shown, any number of members structured to attach an arc to ground
during a phase-to-ground arcing time can be employed. For example
and without limitation, other suitable number of members can be a
flat grounded metal plate, a number of longitudinally oriented
plates, bolts or pins (e.g., single; array), any grounded
conductive or semi-conductive surface (e.g., carbon/graphite), or
any surface that allows the arc to become attached and remain
attached during the arcing time.
Example 1
[0032] FIG. 3 shows another electrical busway 200 including a
plurality of electrical bus members 202,204,206. A plurality of
phase-to-phase arc length limiters 208,210,212 are electrically
connected to the electrical bus members 202,204,206, respectively.
The phase-to-ground arc length limiter 120 is electrically
connected to a housing 222 of an electrical enclosure 224 (as shown
in FIG. 6). The phase-to-phase arc length limiters 208,210,212 are
similar to the phase-to-phase arc length limiters 108,110,112 of
FIG. 2, except that they have a greater length of the cable
terminal pad 240 in order to accommodate a larger number of power
cables, such as 226 (e.g., four example sets of openings 228 are
shown in FIG. 3 (one set being hidden), while there are two example
sets of openings 129 in FIG. 2 (one set being hidden).
[0033] The electrical enclosure 224 of FIG. 6 includes a housing
with a first end 230, an opposite second end 232, and a plurality
of sides 234,236,238,240 disposed therebetween to define an
internal volume 242. The electrical enclosure 224 is further
divided into a plurality of compartments or modules, such as
modules 244,246,248. It will be appreciated that a portion of the
side 240 and the module 248 is not shown in order to show internal
structures such as the example electrical busway 200. Each of the
modules 244,246,248 can be considered to be an electrical enclosure
as employed herein. For example and without limitation, the front
module 244 (to the left of FIG. 6) can include relays, switches,
metering devices, pull fuses, and supplementary protectors; the mid
module 246 (in about the center of FIG. 6) can include circuit
breakers, voltage or control power transformers, fuse trucks,
earthing switches, ground and test devices, and current
transformers; and the rear module (to the right of FIG. 6) can
include electrical busways and electrical devices, such as earthing
switches, rear mounted control power transformers, and lightning
arresters.
Example 2
[0034] Referring again to FIG. 2, the three electrical bus members
102,104,106 are made, for example, of copper, although any suitable
conductor can be employed. Although three example electrical bus
members 102,104,106 are shown, the disclosed concept is applicable
to electrical busways having two or more electrical bus members.
The electrical bus members 102,104,106 can be mechanically
supported by three insulators 131,132,133, respectively, which are
coupled to a bracket 136 (e.g., powder-coated) supported by a side
wall 138 of the housing 122.
[0035] The electrical bus members 102,104,106 include example
copper cable terminal pads 140, although any suitable conductor can
be employed. Power cables 142 (e.g., without limitation, line;
load) are electrically connected to the cable terminal pads 140.
The phase-to-phase arc length limiters 108,110,112 can be made of
zinc chromate plated steel and set the desired phase-to-phase gap
118 (FIG. 4) (e.g., without limitation, about 4 inches; any
suitable gap while still maintaining desired Basic Impulse
Lightning (BIL) requirements; a suitable minimum gap that will not
jeopardize electrical tests per industry standards). The
phase-to-ground arc length limiter 120 can be made of zinc chromate
plated steel and is electrically connected (e.g., without
limitation, bolted; welded; brazed; riveted; clamped; any suitable
mechanism to provide a sufficient preload to maintain a good
electrical path) to the housing 122 (e.g., side channel) of the
electrical enclosure 124 (FIG. 5), which is suitably grounded.
[0036] The phase-to-phase arc length limiters 108,110,112 are
suitably electrically connected to the respective electrical bus
members 102,104,106. However, the placement of the phase-to-phase
arc length limiters 108,110,112 is not limited to be at an
electrical joint in the bus member, but can be located anywhere
along such bus member. For example, these phase-to-phase arc length
limiters could be welded or otherwise suitably electrically
connected anywhere on a continuous piece of an electrical bus
member. For placement of the phase-to-phase arc length limiters
108,110,112 at an electrical joint, there are many factors that
determine a suitable electrical joint/connection (e.g., without
limitation, finish; hardness; preload; surface area). The actual
mechanism for electrical connection depends upon the desired makeup
of the electrical joint.
[0037] The example phase-to-ground arc length limiter 120 includes
four example plates 126, although any suitable number of members,
such as one or more plates, can be employed. The edges 128 of each
of the plates 126 are parallel to each other and are equidistant
(e.g., without limitation, the second gap 130 is about 4 inches;
any suitable gap) from the edges 116 of the phase-to-phase arc
length limiters 108,110,112, which edges 116 are also parallel to
each other.
[0038] The phase-to-phase arc length limiters 108,110,112 are
employed on the respective bus members 102,104,106 and the
phase-to-ground arc length limiter 120 is electrically connected to
the housing 122 of the electrical enclosure 124 (FIG. 5). Placement
of these arc length limiters 108,110,112,120 is done by recognizing
what direction the phase-to-phase arcs 144 (shown in FIGS. 4 and 5)
are anticipated to travel due to, for example, bus geometry,
magnetic force and gas force. By reducing or minimizing the length
of the phase-to-phase arcs 144, subject to design constraints or
design considerations, this provides reduced total arc power, lower
peak pressure, and a "successful pass" of an arc test, such as for
example and without limitation, a 15 kV rated metal clad switchgear
assembly, tested at 63 kA per IEEE Std C37.20.7.TM.-2007 (IEEE
Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for
Internal Arcing Faults).
[0039] Typically, the rear (e.g., in a direction out of the plane
of FIG. 5) of the enclosure 124 (FIG. 5) is toward the right of
FIG. 2. As shown in FIG. 6, a suitably thin insulator 146 (e.g., a
six-sided, insulative boot) can cover the terminal end of each of
the power cables 226 (FIG. 3). The thickness of the insulator 146
is such that the phase-to-phase arcs (not shown in FIG. 3 or 6, but
see the phase-to-phase arcs 144 of FIGS. 4 and 5) are not
obstructed by the insulator 146 during the fault.
Example 3
[0040] As another non-limiting example, the phase-to-phase distance
of the first gap 118 (FIG. 4) can be about 3.5 inches and the
phase-to-ground distance of the second gap 130 (FIG. 2) could range
from about 3.17 inches to about 3.78 inches. However, the actual
distances can be modified depending on a particular design with one
goal of keeping it as close to about 4 inches as possible, although
larger or smaller distances are possible. Primarily, the BIL
requirement prevents a closer spacing; otherwise, the gap 118 could
have about a 1-inch separation.
[0041] For example, for 15 kV rated metal clad switchgear per IEEE
C37.20.2 and IEEE C37.20.7 standards, there is an insulated bus and
the bus joints are insulated with insulative boots, as one example.
For 15 kV rated switchgear, the IEEE standards employ a 95 kV BIL
test. The design parameters to pass this test use about 3-inch
minimum phase-to-phase and phase-to-ground clearances. Slightly
larger gaps as set forth in this example can be employed because
sharp edges are at close proximity. The extra distance is a factor
of safety to pass the 95 kV BIL test.
[0042] For example, the phase-to-phase gap 118 and the
phase-to-ground gap 130 can depend upon meeting suitable design
standards and/or design criteria for a particular product (e.g.,
without limitation, standards, such as NEC, IEEE, CSA or IEC; other
suitable standards; other controlling factor(s)). Preferably, the
gaps 118,130 are as small as possible, but meet the desired design
standards and/or design criteria.
[0043] As another non-limiting example, the phase-to-phase gap 118
and the phase-to-ground gap 130 can vary with voltage. For example
and without limitation, the gaps 118,130 can be relatively greater
at relatively higher voltages (e.g., without limitation, about 7
inches to about 8 inches for 38 kV) and relatively lower for
relatively lower voltages (e.g., without limitation, about 1.5
inches to about 2 inches for 5 kV; about 1 inch for 2.4 kV; less
than 1 inch for 600 V).
Example 4
[0044] In the example of FIG. 3, one of the sides of the enclosure
224 (FIG. 6) supports a bracket 250 carrying a plurality of
insulators 252 (shown in hidden line drawing in FIG. 6), 254 and
256. Each of these insulators supports one of the cable terminal
pads 240 of the electrical bus members 202,204,206.
Example 5
[0045] Referring to FIGS. 2, 4 and 5, the plates 126 of the
phase-to-ground arc length limiter 120 typically show arc
attachment from at least the edge 116 of the center phase at the
center phase-to-phase arc length limiter 110. The outer phases at
the outer phase-to-phase arc length limiters 108,112 can arc at
148,150 to the adjacent side walls 152,154, respectively, of the
electrical enclosure 124 and to the center phase as shown in FIG.
5. The arc fault can start, for example, as a phase-to-phase fault
and then transition to a phase-to-ground fault as the fault current
158 and the convective force motivate, at 157, the arcs 144 upward
and rearward (up and to the right with respect to FIG. 2) to the
phase-to-ground arc length limiter 120 (FIG. 2). Hence, for a
typical arcing fault, the arc length limiters 108,110,112,120 and
the sidewalls 152,154 all show arc attachment.
[0046] As can be seen in FIGS. 2, 4 and 5, the electrical bus
member 102 and the cable terminal pad 140 thereof form a first
L-shape, the electrical bus member 104 and the cable terminal pad
140 thereof form a T-shape, and the electrical bus member 106 and
the cable terminal pad 140 thereof form a second L-shape. The
T-shape of the center phase is disposed between the first L-shape
and the second L-shape of the outer phases. The cable terminal pads
140 of the outer electrical bus members 102,106 form one side of
the first L-shape and the second L-shape, respectively, and are
disposed proximate but separated from the edges 114 of the
corresponding phase-to-phase arc length limiters 108,112.
[0047] The example phase-to-phase arc length limiters 108,110,112
prevent the length of the arcs 144 from significantly increasing
since the J.times.B force 156 directs the arcs 144 toward the
phase-to-ground arc length limiter 120. The arc current density, J,
is equal to the current divided by the arc diameter. The direction
of J is in the direction of current flow in the arc. The magnetic
field, B, direction depends on the direction of current flow but
will generally be enhanced between the gaps between the tops of the
phase-to-phase arc length limiters 108,110,112, to create a force
that pushes the arc outward toward the rear door (e.g., out of the
plane of FIG. 5). The phase-to-phase arc length limiters
108,110,112 reduce the gap between phases. The side connected cable
terminal pad 140 does not produce a magnetic field to drive the arc
downward unlike the configuration of FIG. 1. The phase-to-ground
arc length limiter 120 limits arc length. The configuration of FIG.
2 produces relatively shorter arcs resulting in lower arc voltage
and lower pressure. The J.times.B force 156 (and the upward (with
respect to FIG. 5) convective force 20) advantageously drive the
arcs 144, at 157, toward the phase-to-ground arc length limiter 120
(FIG. 2), which is advantageously disposed somewhat above (with
respect to FIG. 2) and to the rear (to the right with respect to
FIG. 2) of the edges 116 of the phase-to-phase arc length limiters
108,110,112.
[0048] The J.times.B force 156 is produced on the arcs 144 by the
cross-product of the current density (J=fault current
I/cross-sectional area of the arc 144) and the self-produced
magnetic field (B) due to the fault current 158 flowing in the
conductors including the bus member 106 and the phase-to-phase arc
length limiter 112. Unlike normal current flow, a gap (as best
shown in FIG. 4) generally prevents the fault current 158 from
flowing in the cable terminal pad 140. The magnetic field (B) is
additive during part of the time (the current direction depends on
the phase angle of the currents at that instance in time) between
these conductors in the adjacent phases due to the direction of the
current and the orientation of the conductors on either side of the
arc 144. The cross-product is the product of the magnitudes of the
J and B vectors times the sine of the angle (.theta.) between the
vector components of the current density (J) and the magnetic field
(B). In this geometry, the current and magnetic fields are mostly
orthogonal, thereby maximizing the cross-product (i.e.,
.theta.=90.degree.).
[0049] The magnetic force component on the arc is also relatively
stronger than the upward (with respect to FIG. 5) convective force
20. As an example, in the left phase (with respect to FIG. 5)
during an instance in time when the currents are such that the
current path is flowing upward (with respect to FIG. 5) in the left
bus and downward (with respect to FIG. 5) in the center bus, the
fault current path 158 flows up (with respect to FIG. 5) and into
the phase-to-phase arc length limiter 112 and into the arc and back
into the center phase-to-phase arc length limiter 110. The net
movement of the arc 144 by the J.times.B force 156 and the upward
(with respect to FIG. 5) convective force 20 is that the arc 144 is
directed, at 157, toward the phase-to-ground arc length limiter
plates 126 and is subsequently lengthened significantly less than
the arcs 16,18 of FIG. 1, which are moved significantly downward
(with respect to FIG. 1) by the downward magnetic force component
(not shown) since there is no phase-to-phase arc length limiter and
since the power cables 4 are electrically connected to downwardly
extending cable terminal portions as shown in FIG. 1.
[0050] As shown with the electrical bus member 106 of FIG. 4, each
of the phase-to-phase arc length limiters 108,110,112 and the
corresponding electrical bus members 102,104,106, respectively,
direct the fault current 158 flowing therein in the arc 144 between
the adjacent pair of the phase-to-phase arc length limiters
112,110.
[0051] The electrical bus members 102,104,106 include conductors
(e.g., the example cable terminal pads 140) separated from (as best
shown by a gap in FIG. 4) the respective phase-to-phase arc length
limiters 108,110,112. Current normally flows in the corresponding
one of the electrical bus members 102,104,106 and in the conductor
140 separated from the corresponding one of the phase-to-phase arc
length limiters 108,110,112, but not in the corresponding one of
the phase-to-phase arc length limiters 108,110,112. Each of the
phase-to-phase arc length limiters 108,110,112 direct the fault
current 158, as shown with electrical bus member 106 and
phase-to-phase arc length limiter 112, flowing in the corresponding
one of the electrical bus members in the arc 144 between an
adjacent pair of the phase-to-phase arc length limiters 112,110,
but not in the conductor 140.
[0052] The arcs 144 would move horizontally outward (out of the
plane of FIG. 5) if it was not for the convective force 20 that
tends to raise (with respect to FIG. 5) the arcs 144.
Example 6
[0053] Although the electrical enclosure 124 advantageously employs
both of the plural phase-to-phase arc length limiters 108,110,112
and the phase-to-ground arc length limiter 120 (as shown in FIG.
2), the disclosed concept is applicable to configurations that
employ only plural phase-to-phase arc length limiters, such as
108,110,112 (FIG. 5).
Example 7
[0054] Although the electrical enclosure 124 advantageously employs
both of the plural phase-to-phase arc length limiters 108,110,112
and the phase-to-ground arc length limiter 120 (as shown in FIG.
2), the disclosed concept is applicable to configurations that
employ only the phase-to-ground arc length limiter 120. This
limiter 120 can be placed on the sidewalls in addition to above the
arcing area and, in this example, the arcs 144 can be struck
between the sides of the adjacent electrical bus members
102,104,106. However, the limiter 120 can advantageously be placed
anywhere the arc is anticipated to be located.
Example 8
[0055] Table 1 compares peak power, peak pressure, time-to-ground,
and test result for the disclosed concept and for prior
back-connected power cables (FIG. 1) with no arc length
limiters.
TABLE-US-00001 TABLE 1 Peak Peak Time-to- Test Power Pressure
Ground Result Phase-to- 219 MW 18 psig 1.1 mS Passed ground arc
length limiter Back-connected 287 MW 28 psig 2.1 mS Failed power
cables with no arc length limiters
[0056] It will be appreciated that although the disclosed concept
illustrates application of the phase-to-ground arc length limiter
120 together with the phase-to-phase arc length limiters
108,110,112 or 208,210,212, each of these arc length limiters can
be independently employed.
[0057] The phase-to-ground arc length limiter 120 advantageously
limits the upward (with respect to FIGS. 2 and 6) path of the arcs
144 (FIGS. 4 and 5) and, thus, limits the arc length.
[0058] The phase-to-phase arc length limiters 108,110,112 or
208,210,212 use the direction of the fault current 158 to provide a
corresponding J.times.B force 156, which in combination with the
convective force, advantageously drives the arcs 144 toward the
phase-to-ground arc length limiter 120 (FIG. 2).
[0059] The time for the arcs 144 to attach to a ground return is
also greatly reduced by the phase-to-ground arc length limiter 120,
thereby resulting in relatively lower arc power and arc pressure.
Hence, the disclosed concept provides enhanced protection for
electrical enclosures, such as for example and without limitation,
switchgear enclosures.
[0060] It will be appreciated that any phase-to-phase arc length
limiter or any phase-to-ground arc length limiter as disclosed
herein can be suitably formed and/or electrically connected to a
corresponding electrical bus member or an electrical enclosure
housing, respectively, by any known or suitable mechanism or
method, including, but not limited to, welding.
[0061] While specific embodiments of the disclosed concept have
been described in detail, it will be appreciated by those skilled
in the art that various modifications and alternatives to those
details could be developed in light of the overall teachings of the
disclosure. Accordingly, the particular arrangements disclosed are
meant to be illustrative only and not limiting as to the scope of
the disclosed concept which is to be given the full breadth of the
claims appended and any and all equivalents thereof.
* * * * *